1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, it relates to an active matrix liquid crystal display device of a so-called in-plane switching type.
2. Description of the Related Art
A liquid crystal display realizes display in such a manner that an electric field is applied to liquid crystal molecules in a liquid crystal layer sandwiched by a pair of substrates to change the orientation direction of the liquid crystal, so as to cause optical change of the liquid crystal layer.
The conventional active matrix liquid crystal display device is represented by a twisted nematic (TN) display system, in which the direction of the application of the electric field to the liquid crystal is set in the direction perpendicular to the substrate plane that sandwiches the liquid crystal, and the display is realized by utilizing the optical rotation of the liquid crystal layer.
On the other hand, a liquid crystal display device of a in-plane switching (IPS) system has been proposed in JP-B-63-21907, U.S. Pat. No. 4,345,249, WO 91/10936 and JP-A-6-160878, in which a comb electrode is used, and the direction of the electric field applied to the liquid crystal is set in the direction parallel to the substrate plane, whereby the display is realized by utilizing the birefringence of the liquid crystal.
The in-plane switching system has advantages, such as a wide viewing angle and a low load capacitance, in comparison to the conventional TN system, and is being rapidly developed in recent years as a new active matrix liquid crystal display device that superseding the TN system.
In the IPS system, the in-plane switching can be more perfectly realized in the case where the liquid crystal has negative dielectric anisotropy in comparison to the case of a liquid crystal having positive dielectric anisotropy, as demonstrated in J. of Appl. Phys., vol. 82, No. 4, pp. 528-535 (1997) by M. Oh-e, M. Yoneya and K. Kondo. The liquid crystal having negative dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is larger than the dielectric constant in the long axis direction perpendicular thereto, and the liquid crystal having positive dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is smaller than the dielectric constant in the long axis direction perpendicular thereto.
The perfect realization of the in-plane switching completes enhancement of the viewing angle of the liquid crystal display device including halftone. Therefore, the liquid crystal having negative dielectric anisotropy is preferred as a liquid crystal used in the IPS system from the foregoing standpoint.
The IPS system employs an opaque metallic comb electrode in a stripe form provided on an inner surface of one of the pair of electrodes.
In recent years, a modified system of the IPS system has been proposed in that the comb electrode is formed with a transparent electroconductive substance, such as ITO (indium tin oxide), instead of the opaque metallic electrode, and is arranged at a shorter interval than the conventional IPS system, and a liquid crystal material having negative dielectric anisotropy, whereby the entire liquid crystal present above the transparent comb electrode can be subjected to orientation change only with an electric field formed at the periphery of the comb electrode, so as to improve the transmittance and the opening ratio.
Literatures relating to the foregoing proposal include Asia Display 1998, pp. 371-374, by S. H. Lee, S. L. Lee and H. Y. Kim and SID digest 1999, pp. 202-205, by S. H. Lee, H. Y. Kim and T. Y. Eom.